Key electrical switchboard at Shika nuke plant almost flooded: NRA

The Shika Nuclear Power Plant is seen in Shika, Ishikawa Prefecture, in this Nov. 2, 2011, file photo. (Mainichi)

A key electrical switchboard at Shika Nuclear Power Plant in Ishikawa Prefecture was nearly flooded after rainwater flowed into one of the plant's reactor buildings last month, Japan's nuclear regulator has revealed.

The Nuclear Regulation Authority (NRA) instructed plant operator Hokuriku Electric Power Co. to draw up a report illustrating preventive measures on the grounds that the incident could have led to a serious accident.

"We'd like to raise our awareness of safety by learning lessons from the trouble," Hokuriku Electric President Yutaka Kanai told NRA officials on Oct. 19.

An area around the atomic power station in Shika, Ishikawa Prefecture, had about 30 millimeters of rainfall per hour on Sept. 28, flooding roads on the plant grounds, according to the NRA. Some 6.6 metric tons of rainwater flowed into the first floor and basement of the No. 2 reactor building through an underground passage for temporary cables, causing an electrical switchboard for lighting to short out.

The accident is attributable to the volume of rainwater exceeding the capacity of a drainage pump in the building, to a gap in the lid covering the underground passage, and to the plant operator's failure to repair a crack in the floor of the reactor building.

There are important systems in the flooded area, including an electrical switchboard that supplies power to an emergency reactor cooling unit, and emergency batteries. If these devices had been submerged, they could have become inoperable.